Add faulty circle_area function and tests

tests/test_circle_area.py

@@ -0,0 +1,15 @@
+from unittest import TestCase
+import math
+
+from drone_test import circle_area
+
+class CircleAreaTest(TestCase):
+    def test_area_1(self):
+        self.assertEqual(circle_area(1), math.pi)
+
+    def test_area_2(self):
+        self.assertEqual(circle_area(2), math.pi * 4)
+
+    def test_negative(self):
+        with self.assertRaises(ValueError):
+            circle_area(-1)